1. Field of the Invention
The invention relates to a method for coating high quality thin film magnetic disks for the magnetic storage of data--known as storage disks--by magnetron sputtering.
2. Description of Related Art
Magnetic storage disks in general have a substrate comprising aluminum alloy or glass. To this substrate, a coating set comprising an underlayer, preferably of chromium or a chromium alloy such as CrV or CrTi, a magnetic storage layer comprising a cobalt alloy, such as, for example, CoCrNi, CoCrTa or CoCrPt, and a thin protective and lubricating layer, generally carbon, are typically applied. Glass substrates are typically coated with an additional thin layer under the underlayer which is called a seed layer. In the following the term substrate will be used to include the optional seed layer if present.
The demand for increase in storage density to values greater than 100 Mbit/inch.sup.2 requires the application of magnetic layer sets with a high coercive field strength. For this the layer thickness must not be increased but must, on the contrary, be reduced. Typical values for the coercive field strength have been from 1500 to 2000 Oe. More recent developments for storage disks demand an increase in this value to over 2000 Oe with a high reproducibility of the manufacturing process.
It is known in the prior art that one can apply the requisite layers of the layer set by direct current magnetron sputtering using targets consisting of the alloys to be deposited, e.g. chromium targets can be used to deposit a thin film chromium layer. A target in the terminology used in the sputtering art refers to the replaceable solid elements which are bombarded by the process gas ions to liberate ions which migrate to the substrates. The power source on which the targets are mounted is called the "source." Investigations carried out on methods of improving the layer properties show two approaches in developments to raise the coercive field strengths of sputtered layers for magnetic disks: a) increase of the substrate temperature during coating and b) the use of bias voltage on the substrate in depositing the underlayer and the magnetic layer (cf. Noriaki Tani et al., "Increase of Coercitive Force in Sputtered Hard Disk", IEEE Transactions on Magnetics, Vol. 26, No. 4, July 1990 and S. Schulz et al., "A new single disk coating system for magnetic disks", Society of Vacuum Coaters, 35th Annual Technical Conference Proceedings (1992), pp.382-387).
There are, however, practical limits on the use of these teachings and they have the following disadvantages: the upper limit of the substrate temperature is limited to 200 to 230.degree. C. by the mechanical properties of the substrate material and the substrate mount. Even when glass substrates are used these limits cannot be greatly exceeded. In the case of moving substrates the application of a bias voltage creates technical problems. In the transition to glass substrates the use of direct current biasing is essentially not possible. Thus, a bias voltage is rarely used. Accordingly, the potential of the layer materials can thus not be fully exploited in accordance with the state of the art.